It's 10-20% according to most estimates, enough to flood every hospital system in the world. Once that happens the death toll starts creeping towards 20% as they can't treat patients and even people otherwise treatable problems can't get treated. This is the scenario that needs to be avoided.

> Vast majority of people who contract the virus just have mild cold or flu symptoms.

Mild in the medical sense, everyone not hospitalized is considered mild. In the everyday human sense this translates from nothing to extremely sick. If your friend had pneumonia you'd be concerned for them wouldn't you? Well that's a "mild" case.